ARIZONA

The
Lost Nummel Gold Mine

GEOLOGY OF THE AREA

The Chocolate Mountains and the adjoining Trigo Mountains
of southwest Arizona are part of the Basin and Range Province
of North America. This physiographic province comprises the
southern third of the state plus its western edge. Most of
the mountain ranges in the Basin and Range Province trend
north-south or northwest- southeast and consist of uplifted,
fault-bounded blocks (known as horsts) separated from each
other by downthrown, alluvium-filled basins (known as grabens).
In southern Arizona, ancient Precambrian basement rock is
sometimes exposed in the cores of mountain ranges. Interestingly,
many of these Precambrian "metamorphic core complex" ranges
trend northeast-southwest, marking them as somewhat unique
in Basin and Range geology. Examples in southern Arizona include
the Buckskin Mountains, Harcuvar Mountains, Harquahala Mountains,
Buckeye Hills and the White Tank Mountains. All of these mountain
ranges trend northeast-southwest and are made up almost entirely
of ancient Precambrian granite gneiss. This is the oldest
rock in Arizona.

Most of the ranges in southern Arizona have a younger igneous
origin. Four major volcanic "pulses", beginning in the middle
of the Mesozoic, are recognized. These episodes of volcanism
produced massive amounts of lava, hypabyssal rocks, tuffs,
and agglomerates. The majority of the mountain ranges in southern
Arizona are therefore made up of early-Cretaceous intermediate
volcanics (andesites), younger Laramide-age volcanic dikes
and granite plutons, younger still mid-Tertiary volcanics,
or fairly recent Quaternary basalts. This is true of the Chocolate
Mountains and the adjoining Trigo Range. Most (but not all)
of southern Arizona's mineral deposits are products of Laramide
mountain-building processes.

The highly-weathered Chocolate Mountains (and the adjoining
Trigo Mountains) are predominantly composed of Cretaceous-age
andesite flows, tuffs, and agglomerates. These older volcanics
make up most of several mountain ranges in southwestern Arizona,
including the Middle Mountains, the Kofa Mountains, and the
southern half of the Plomosa Mountains. In this respect, the
Chocolate Mountains are similar to the majority of the desert
ranges of southern Arizona. However, they differ in that they
trend roughly northeast-southwest.

The Chocolate Mountains merge with the north-south trending Trigo
Mountains at their southernmost points. Here, Yuma Wash separates the southern spurs
of the two ranges. Rock type in this area is mostly andesite. A small exposure (less than
1 square mile in outcrop area) of Laramide volcanics occurs about 8 miles northeast of
the mouth of Yuma Wash, in the Chocolate Mountains. The Trigo Mountains are home
to the Red Cloud mining district (the Red Cloud Mine Road enters the mountains via
Yuma Wash). The mineralized zone at the Red Cloud Mine lies only about 5 to 6 miles
northwest of the mouth of Yuma Wash.

Although most of the Chocolate Mountains and Trigo Mountains are
composed of Cretaceous andesites, some exposures of slightly older Mesozoic schists and
gneisses occur in the area. Indeed, the largest body of Mesozoic schist makes up the
southern spur of the Trigo Range and therefore forms the western boundary for much of
the lower sections of Yuma Wash.

PROSPECTING POTENTIAL

The
area of interest for John Nummel's lost gold ledge includes
the southernmost portions of the Trigo Mountains and the Chocolate
Mountains. Indeed, modern-day prospectors can still find placer
gold in the gullies, gravel bars, gravel terraces, and around
old placer diggings throughout most of the southern portions
of both ranges. Yuma Wash basically lies between the two ranges
at their southernmost points. The area is highly mineralized.
The southern Trigos contain many silver mines including the
famous Red Cloud

Mine, Papago Mine, Black Rock Mine, and Clip Mine. Prospectors
should probably concentrate on the canyons in and around Yuma
Wash. This area includes the southernmost flanks of the Trigo
and Chocolate Mountains.